Every class has about 120 weapons they can get, but many of them are evolutions and every one has a Primordial version which is even stronger. So final count for all classes is about 240 weapons each.
I thought I was going well with one character but I am really only started... getting the basic versions is only the beginning. Not all weapons evolve, although the screen that lists them all will show a link to the next reaper if it does.
For instance, the top reaper here has 6 evolutions but its the only one I have that has that many. Most of that page don't evolve into another staff, other pages are better for that. Evolutions don't replace previous species of reaper.
I am missing the Primordial reaper tab but as I am still unsure how to upgrade them to Primordial I won't worry about it yet. The weapon o dex includes all three classes so there aren't 15 pages of just Staffs.
You get them via the Great Forge AKA the endless dungeon. I am still a little hazy about what I need to do to upgrade them? Hard to find clear info on game... google search and watching videos appears to be only answer.
Also, when I say 240, they aren't different entirely. They are different versions of the same effects. So there is a sword, Staff and Bow of Unity, for instance. One of the shops in town lets you make any weapon you have found the alternative of, If I play long enough I will get to that process, I just lack the mats. So me finding all the Staffs might pay off for the Knight or Huntress later on.